A controlled post-synthesis route to well-defined and active titanium Betaepoxidation catalysts

This paper describes the successful indirect synthesis of a titanium Beta z eolite. The post-synthesis modification, consisting of separate dealuminati on and titanation steps, is extensively studied by X-ray diffraction, Fouri er-transformed infrared (FT-IR), X-ray photoelectron spectroscopy, diffuse reflectance UV-Vis and Raman spectroscopy. Dealumination degrees up to 90% could be obtained by using oxalic acid as well as nitric acid without any l oss in crystallinity or micropore volume. FT-IR shows the formation of sila nol nests upon removal of aluminum from the zeolite framework. Subsequently , these silanol nests react with titanium(IV) chloride in the titanation st ep, resulting in tetrahedral titanium sites. The post-synthesis procedure d escribed allows the preparation of titanium zeolites that are free from TiO 2. The results of the titanation experiments suggest that the vacant sites are first filled with titanium, resulting in tetrahedral, isolated titanium sites, before the formation of TiO2 starts to occur. As such, the titanium loading can be tuned to about 2 wt.% titanium and, at the same time, the f ormation of TiO2 species is prevented. The titanium Beta materials obtained are active and truly heterogeneous epoxidation catalysts. Both with tert-b utyl hydroperoxide (under dry conditions) and with aqueous H2O2, they effec tively catalyze the epoxidation of alkenes under mild conditions. It is dem onstrated that TiO2-free catalysts are highly preferable, since traces of T iO2 present on the titanium zeolite impair the catalyst performance. In the epoxidation of 1-octene, the Ti-Beta catalysts prepared via the controlled post-synthesis route can easily compete with TS-1 under the same condition s. (C) 1999 Elsevier Science B.V. All rights reserved.